A pressure swing adsorption process as hereinabove described is known from European patent specification 0 092 695 for the purification of helium, wherein starting with a mixture containing helium and consisting essentially of nitrogen, argon and oxygen, as well as smaller fractions of carbon dioxide and methane, and using molecular sieves, helium with a purity of over 99.9% by volume can be obtained. However, the initial mixture in this process already has a content of 50-95% by volume of helium. This process is not suitable for gas mixtures containing only up to 10% helium, since the process cycle and the combination of process steps are not suitable.
From U.S. Pat. No. 3,636,679 a further pressure swing adsorption process for helium enrichment is known, wherein the gas mixture is cyclically fed to 4 adsorbers, each of them passing successively through a pressure build-up phase, an adsorption phase and a pressure relief phase, whereby the pressure build-up and the pressure relief are partially performed through pressure compensation with two different adsorbers, the pressure build-up phase having three steps, and the pressure relief phase having four steps. However, in this case, besides the pressure relief steps an additional flushing step is required for a complete regeneration. The product gas yield (according to Example 53%) is therefore comparatively low. Furthermore, it is disadvantageous that the product gas recovery takes place at low process pressure (during the pressure relief steps), so that the product gas has to be brought to a higher pressure level for many purposes of use. Moreover, the various pressure levels, at which the product is obtained, require a pressure compensation, which again can lead to oscillations in the amount of product gas.
Helium is increasingly in demand for several applications, e.g. refrigeration plants for refrigeration, as a shielding gas during welding and in the chemical industry, as inert gas in space technology, as a respiration gas during diving, as a carrier gas in chromatography, for the detection of leakages, as a balloon-filling gas and for other purposes as well. For these purposes, helium is required with a high degree of purity. In order to achieve this high purity level, in gas mixtures containing only low levels of helium, several process steps are required, in order to first enrich the gas mixture with helium and then to recover high purity helium from this helium-enriched gas mixture.
Helium is enriched and recovered mainly from helium-containing natural gases. The main components of these gases are nitrogen and methane, as well as up to 10% by vol. helium, besides lower proportions of several higher-molecular weight hydrocarbons and carbon dioxide.
According to the state of the art, the following method of helium enrichment is known, as has been published in "Bureau of Mines, Preprint from Bulletin 675--Helium--1985 Edition, U.S. Department of Interior", pages 3 and 4.
The helium-containing natural gas is cooled down to approx. -150.degree. in a cryogenation plant, whereby primarily the hydrocarbons will be released by condensation. The so-produced gas mixture, except for low proportions of other gases contains more than 50% by vol. helium and nitrogen. Such crude helium can be treated on the spot to give a helium of very high purity, e.g. by subjecting it to some process combination comprising a pressure swing adsorption plant plus a second cryogenation unit.
Another alternative is to sell the crude helium as an intermediate product to be treated by some third party.